Magnetic detector

ABSTRACT

A probe for detecting magnetic particles. In one embodiment, the probe includes: a cylindrical probe core having a first end and a second end, the cylindrical probe core defining two channels for containing coils of wire, one of the channels being adjacent the first end of the cylindrical probe core; two sense coils, one each of the sense coils being located in a respective one of the channels; and two drive coils, one each of the drive coils being co-located with the respective sense coil in a respective one of the channels.

RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.13/799,334, filed on Mar. 13, 2013, the entire disclosure of which isincorporated herein by reference.

This application is related to U.S. patent application Ser. No.13/799,480, filed on Mar. 13, 2013, the contents of which areincorporated by reference in their entirety.

FIELD OF THE INVENTION

The invention relates generally to the field of medical devices forlocating tissue in preparation for surgery and more specifically fordetecting magnetic markers in tissue for excision.

BACKGROUND

The recent use of magnetic sensor probes to detect magneticnanoparticles in the localization of sentinel lymph nodes in the stagingof cancer in preparation for surgery has made the job of determining thelocation of the sentinel nodes easier for the surgeon. Further, the useof a probe to detect magnetic markers has also made relocating biopsysites easier after pathology microscopic examination of excised tissue.

The inventors of the sensor probes for these systems seek to improve thedesign by: reducing thermal effects which cause coils in the sensor toshift with respect to one another and reduce the ability of the user todetect the signal from the magnetic nanoparticles; reducing interferencecaused by diamagnetic responses due to the body itself; and reducing theinterference caused by eddy currents induced in objects near the sensorprobe. In addition, it is desired that all these functional improvementsbe accomplished using a smaller sized probe with heightened sensitivity.

The present invention addresses this need.

SUMMARY OF THE INVENTION

In one aspect, the invention relates to a probe for detecting a magneticmarker. In one embodiment, the probe includes a probe core having afirst end and a second end, the probe core defining two regions forcontaining coils of wire, one of the regions being adjacent the firstend of the cylindrical probe core; two sense coils, one each of thesense coils being located in a respective one of the regions; and twodrive coils, one each of the drive coils being located in a respectiveone of the regions, wherein the regions are separated by a distanceequal to or greater than the diameter of one of the coils. In anotherembodiment, the magnetic marker comprises magnetic nanoparticles. In yetanother embodiment, one of the set of two drive coils and the set of twosense coils is connected as a gradiometer, and the other of the set oftwo drive coils and the set of two sense coils is connected in series.In still yet another embodiment, the regions of the probe core definetwo channels for containing coils of wire, one of the channels beingadjacent the first end of the cylindrical probe core, wherein one eachof the sense coils being located in a respective one of the channels,wherein one each of the drive coils being co-located with the respectivesense coil in a respective one of the channels, wherein the drive coilsare connected in series, and wherein the sense coils are connected inanti-series.

In one embodiment, the drive coil is wound on top of the sense coil. Inanother embodiment, the regions of the probe core define two channelsfor containing coils of wire, one of the channels being adjacent thefirst end of the cylindrical probe core, wherein one each of the sensecoils being located in a respective one of the channels, wherein oneeach of the drive coils being co-located with the respective sense coilin a respective one of the channels, wherein the drive coils areconnected in anti-series, and wherein the sense coils are connected inseries. In yet another embodiment, the regions of the probe define fourchannels for containing coils of wire, a respective two of the channelsbeing located in each of the respective regions, wherein two of thechannels are located adjacent the first end of the cylindrical probecore, wherein one each of the sense coils being located in a respectiveone of the channels in each one of the regions, wherein one each of thedrive coils being located in a respective one of the channel in each oneof the regions, wherein no two coils occupy the same channel, whereinthe drive coils are connected in series, and wherein the sense coils areconnected in anti-series.

In one embodiment, the regions of the probe define four channels forcontaining coils of wire, a respective two of the channels being locatedin each of the respective regions, wherein two of the channels arelocated adjacent the first end of the cylindrical probe core, whereinone each of the sense coils being located in a respective one of thechannels in each one of the regions, wherein one each of the drive coilsbeing located in a respective one of the channel in each one of theregions, wherein no two coils occupy the same channel, wherein the drivecoils are connected in anti-series, and wherein the sense coils areconnected in series. In another embodiment, the order of the coils fromthe first end toward the second end of the probe is sense coil, drivecoil, sense coil, and drive coil. In still yet another embodiment, theorder of the coils from the first end toward the second end of the probeis drive coil, sense coil, drive coil, and sense coil.

In one embodiment, the order of the coils from the first end toward thesecond end of the probe is sense coil, drive coil, drive coil, and sensecoil. In yet another embodiment, the order of the coils from the firstend toward the second end of the probe is drive coil, sense coil, sensecoil, and drive coil. In still yet another embodiment, the sense coilsand the drive coils have different diameters. In another embodiment, thesense coils and the drive coils have different numbers of turns.

In one embodiment, the probe further includes a third drive coil, thethird drive coil in series connection between the first and the seconddrive coil and positioned between the first and second drive coil so asto form a single solenoidal drive coil, wherein one each of the sensecoils being located near a respective end of the single solenoidal drivecoil. In another embodiment, every turn of the single solenoidal drivecoil is of the same diameter. In yet another embodiment, thelongitudinal center turn of the single solenoidal drive coil has agreater diameter than turns on the ends of the solenoidal drive coil. Instill another embodiment, every turn of the single solenoidal drive coilis of the same spacing. In still yet another embodiment, turns in thelongitudinal center of the single solenoidal drive coil have a greaterspacing than turns on the ends.

In one embodiment, the probe core comprises a material with a thermaldiffusivity of substantially ≧20×10⁻⁶ m²/s and a thermal expansioncoefficient of substantially <3×10⁻⁵/° C.

In another embodiment, wherein the probe core comprises a material thathas a thermal diffusivity of substantially ≧50×10⁻⁶ m²/s and a thermalexpansion coefficient of substantially <5×10⁻⁶/° C.

BRIEF DESCRIPTION OF THE DRAWINGS

The structure and function of the invention can be best understood fromthe description herein in conjunction with the accompanying figures. Thefigures are not necessarily to scale, emphasis instead generally beingplaced upon illustrative principles. The figures are to be consideredillustrative in all aspects and are not intended to limit the invention,the scope of which is defined only by the claims.

FIG. 1 is a block diagram of an embodiment of a probe constructed inaccordance with the invention;

FIG. 2 is a block diagram of another embodiment of a probe constructedin accordance with the invention;

FIG. 3 is a cross-sectional diagram of the embodiment of the probe ofFIG. 2;

FIG. 4 is a block diagram of another embodiment of a probe using asolenoid coil constructed in accordance with the invention;

FIG. 5 (a-d) are cross-sections of four solenoid coil configurations;

FIG. 6a is a cross section of a solenoidal coil with additional coils oneach end;

FIG. 6b is a graph of the field generated by the solenoidal coil of FIG.6 a;

FIG. 7 is a cross-sectional representation of a Helmholtz coilembodiment;

FIG. 8 is a graph of the modulation of a primary magnetic field with asecondary magnetic field and the resulting frequency spectrum;

FIG. 9 is an embodiment of a secondary field generator for use increating the secondary magnetic field shown in FIG. 8; and

FIG. 10 is an embodiment of a secondary field generator for use increating the secondary magnetic field shown in FIG. 8.

DESCRIPTION OF AN EMBODIMENT OF THE INVENTION

First, to improve the sensitivity of the coil while reducing its size,it is important to reduce any thermal effects on the operation of theprobe, which is highly sensitive to relative change in geometry betweenthe coils, particularly axial movement, and any change in the size ofthe coils. One of the thermal effects which must be addressed is anyasymmetric expansion of the coil arrangement when the end of the probecomes into contact with a warm body, and the coil nearest to the bodyexperiences a higher temperature than the coil further away. Reducingthe temperature differential between the coils, while maintaining thelow thermal expansion coefficient of the coil former or probe body,permits this sensitivity to be reduced. In other words, it is importantto equalize any temperature differential across the coils rapidly sothat a thermal gradient cannot be sustained by a thermal input at oneend.

When a substance has high thermal diffusivity, heat moves rapidlythrough the substance because the substance conducts heat quicklyrelative to its volumetric heat capacity or “thermal bulk.” Therefore, amaterial with high thermal diffusivity is desirable for the coil former(probe core material) in order to equalize the temperature across thecoils as rapidly as possible. Further, suitable materials need to benon-magnetic, non-electrically conducting and have a relatively lowthermal expansion coefficient. The low thermal expansion coefficient isrequired to reduce axial and radial expansion which affects the relativepositions of the coils. Table 1 shows relevant properties of variousmaterials.

In one embodiment, the material preferably has a thermal diffusivity ofsubstantially ≧20×10⁻⁶ m²/s and a thermal expansion coefficient ofsubstantially <3×10⁻⁵/° C. More preferably, the material has a thermaldiffusivity of substantially ≧50×10⁻⁶ m²/s and a thermal expansioncoefficient of substantially <5×10⁻⁶/° C. Such materials may include:

-   -   Glassy ceramics such as borosilicate based machinable ceramics        e.g. Macor® (Corning Inc, New York);    -   Non-glassy ceramics such as aluminum nitride, boron nitride,        silicon carbide;    -   Composite ceramics such as Shapal-M, a composite of boron        nitride and aluminum nitride (Tokuyama Corporation, Shunan City,        Japan); and    -   Carbon- and glass-filled composites, for example glass or carbon        filled Polyetheretherketone (PEEK).

Further, it is advantageous for the material to have a high stiffness toavoid change in the position of the coils due to mechanical deformation.In one embodiment, the material has a Young's Modulus of ≧40 GPa andpreferably ≧80 GPa, and the material has as high a toughness as can beachieved given the other material constraints. For ceramics, in thisapplication, the likely failure mode is energy- (or shock-) inducedbrittle fracture, for example when the probe is knocked or dropped. Inthis case, the relevant material property is the toughness G_(ic)≈K_(ic)²/E (KJm⁻²) which, along with other probe information, is tabulated inTable 2.

The probe's sensitivity to temperature changes is further reduced byminimizing heat transfer into the probe from outside of the probe. Thisis achieved by using a high conductivity polymer material for the case,so that the inside surface of the case is closer to an isothermalsurface, thus minimizing thermal gradients inside the probe andproviding an insulating layer of air or a highly insulating materialsuch as aerogel or a vacuum gap around the probe core, or former, andwindings. In addition to the selection of materials, the coilconfigurations may be selected to reduce thermal effects.

In one embodiment, the probe includes a coil former with two sense coilsand two drive coils. The sense and drive coils are arranged in twopairs, each consisting of one sense coil and one drive coil in closeproximity to one another. The probe's magnetic sensing performance ismaximized by locating one sense and drive coil pair close to the sensingtip of the probe, and locating the second pair of coils an axialdistance away from the first pair of coils. The distance is preferablygreater than the diameter of the smallest coil, and more preferablygreater than the diameter of the largest coil in the pair. By way ofexample, in one embodiment, the diameter of the largest coil is 15 mm,and in another embodiment, the diameter is 12 mm. In addition, each pairof coils is arranged such that the voltage induced in each of the sensecoils by the field generated by the drive coils is approximately equaland opposite. For example, if there are two sense/drive coil pairs S1,D1 and S2, D2, separated by at least a coil diameter, the currentinduced in S1 by the combined drive fields from D1 and D2 is equal andopposite to the current induced in S2 by the combined drive fields fromD1 and D2. Preferably, the sense coils are arranged as a first ordergradiometer in order to minimize the effect of far field sources.

In one embodiment of the probe 4 (FIG. 1), the coils 10, 10′, 16, 16′are co-located such that each drive coil 10, 10′ is co-located with asense coil 16, 16′. Either order of the arrangement of the sense anddrive coils can be used. In one embodiment, the drive coil is wound ontop of the sense coil in order to minimize any effect from thermalexpansion of the drive coils as the drive coils warm due to ohmicheating. This arrangement is useful where thermal effects are lessimportant than sensitivity at very small diameters (e.g. <15 mm).Because this arrangement does not require specific diameters and spacingof the drive and sense coils, it is suitable for use in probes of a verysmall outer diameter. In one embodiment, this probe has a diameter ofless than 15 mm or even less than 10 mm in diameter.

In another embodiment of the probe 6 (FIG. 2), drive 10, 10′ and sense16, 16′ coils are staggered with the drive coils 10, 10′ being separatedand the sense coils 16, 16′ are divided into two parts. The spacing ofthe drive coil relative to the sense coil in each pair is chosen suchthat the effects of the expansion of the larger of the two coils on themutual inductance of the pair of coils is minimized. This helps furtherto minimize the effect of thermal expansion. This arrangement providesimproved sensing because the sensitivity drops off more slowly withdistance. In this arrangement, the sense coil is closest to the sensingend of the probe in order to maximize sensing distance. In thisarrangement, from the sensing end, the coil order is SD-SD where Drepresents a drive coil and S represents a sense coil. Referring to FIG.3, a cross-section of an embodiment of a probe 20 is shown including ahousing 24, temperature barrier 26 and a coil former or probe core 30having circumferential grooves 28 into which the coils 10, 10′, 16. 16′are formed.

Other embodiments which maintain the relative arrangement of eachdrive/sense pair also fall within the scope of the invention. Theseembodiments include different orders and symmetry of the pairs, e.g.SD-DS, DS-DS and DS-SD, as well as varying spacing between the pairs ofcoils. In each case, the coils are connected such that one of the drivepair of coils or the sense pair of coils forms a gradiometer. The otherpair of sense coils or drive coils is wired in series, i.e. with thesame sense to form a simple magnetometer. A higher order gradiometer canbe used for either set of coils, providing that in each case the orderof gradiometer of the drive set of coils differs by one from the orderof gradiometer of the sense set of coils. For example, the sense coilsmay form a second order gradiometer and the drive coils may form a first(or third) order gradiometer.

In one embodiment, the winding is arranged together with the electroniccircuit such that the outside layer of the coil winding is close toground potential of the electronics. This minimizes the capacitivecoupling between the probe and the patient's body which is assumed to beat ground potential.

Further embodiments include making the sense coil the larger of the twodiameters (rather than the drive coil) in combination with any of theabove. If a smaller gauge wire is used for the sense coil (as it carriesonly a tiny current), then making the sense coil the larger of the twois advantageous because it allows the average diameters of the drive andsense coils to be maximized within the specific arrangement of the sensecoil/drive coil pairs. Increasing the diameter and therefore the areasof the coils increases the sensitivity of the magnetic sensor. A largergauge of wire may be used for the drive coil in order to minimize ohmicheating.

Further, when the diameter of the probe is reduced, the magneticsensitivity is commensurately reduced by a factor of r⁴ in the nearfield (drive field drops off with r² and sensing capability with r²) andr⁶ in the far field. Therefore, in order to maintain a similar level ofmagnetic sensitivity in a smaller diameter probe, the number of turns inthe coils, particularly in the sense coils, is increased. However, as aside effect, this also magnifies any noise or drift by the same factor.Thus, drift due to changes in the coil geometry caused by temperaturechanges should be expected to be magnified.

However, by using the staggered coil arrangement and the Shapal-M coilcore, the thermal drift due to warm body contact can be maintained at anacceptable level. For example, with a smaller diameter probe of thestaggered coil arrangement, the signal change in response to contactwith a warm body at 37° C. is about 86% of the equivalent signal changein one of the prior art systems. Because the smaller probe has twice asmany coil turns, the signal change should therefore be much greater thanthis.

In a further embodiment (FIG. 4), a solenoid 31 is used as theexcitation (drive) coil and two sense coils 34 are positioned inside thesolenoid 31. As long as the sense coils 34 are not too close to the endof the solenoid 31, they will experience a substantially uniformmagnetic field from the solenoid 31. The benefit of this arrangement isthat small movements relative to the solenoid 31, for example due tothermal expansion, will not disturb the (magnetic) balance of the sensecoils 34. Further, the solenoid will also provide an additional heatconduction path to the sense coils and the coil core to help equalizetemperature across all the coils. By choosing the wire gauge, the ohmicheating effect of the solenoid can be minimized. The solenoid coil alsoforms an electrostatic shield for the sense coil.

The more uniform the field is close to the ends of the solenoid, thecloser the sense coils can be positioned to the end of the probe, andthe better the sensitivity. The uniformity of the field at the ends ofthe solenoid is optimized by appropriate design of the coils, forexample, by varying the spacing, the diameter, both the spacing anddiameter, or the shape of the solenoid coils (FIG. 5 (a-d)). Design andmanufacturing of such solenoids is known to those skilled in the art.

A particular example is shown in FIG. 6a , together with a graph (FIG.6b ), showing the results of modeling the resulting normalized magneticfield strength. By adding two coils 40, 40′ at the end of the solenoid44, the field uniformity along the length can be improved (46) over thefield of the solenoid alone (50). A special case of the solenoid drivecoil is one in which the drive coil is formed from a Helmholtz pair ofcoils where the separation between the coils, 1, is substantially equalto the radius of the coils (r) (FIG. 7). The Helmholtz coil is awell-known arrangement that provides a region of constant magnetic fieldbetween the two coils, and this region extends further than in any otherarrangement of the coils with a different ratio of separation-to-radius.

In addition to manipulating the coils and their structure and locations,a further embodiment addresses the issue of distinguishing between thesignal from the iron oxide nanoparticles and the signals from othermetallic objects. In this embodiment, a second varying magnetic drivefield is generated at a lower frequency than the primary drive field andwith comparable or greater magnitude than the primary drive field. Thesecondary drive field modulates the susceptibility of the magneticnanoparticles and creates additional frequency components in thespectrum received by the sense coils at frequencies f₀±2nf₁, where f₀ isthe primary drive field frequency and f₁ is the secondary drive fieldfrequency and n is a whole number (FIG. 8). The presence of magneticnanoparticles can be detected by analyzing these additional frequenciesthat are a result of the mixing of the primary and secondary drivefrequencies.

In one embodiment, the amplitude of the side lobe at f₀±2f₁ is measuredto detect the presence of magnetic nanoparticles. In an alternativeembodiment, the ratio of the amplitude A_(f0±2f1) of the side lobe tothe amplitude A_(f0) of the fundamental (A_(f0±2f1)/A_(f0)) is measured.Other embodiments are possible that result in an advantageouscombination of frequencies. These components are less sensitive or eveninsensitive to coil structure and geometric imbalance, temperatureeffects, and also to eddy currents. Thus, by appropriate signalprocessing, the undesirable disturbance from coil structure andgeometric imbalance, temperature sensitivity, and metallic objects canbe reduced or eliminated.

In practice, the geometrical symmetry of construction needed for aperfect electromagnetic balance is very difficult to achieve within theconstraints of state-of-the-art and cost-effective construction, andthere is usually some level of residual imbalance. Furthermore,temperature fluctuations experienced by the coils, particularlynon-uniform changes, for example when the end of the probe (near onecoil) comes into contact with a warm body, will create additionaldynamic imbalance in the coils as the geometry of the coils variesslightly due to thermal expansion.

These imbalances in the coils manifest themselves as a disturbancesignal at the fundamental primary drive frequency f₀, caused by thesense coil detecting a non-cancelled residual direct magnetic signalfrom the drive coils. However, any signal at a mixed or modulatedfrequency f_(0±2)nf₁ resulting from the secondary frequency f₁ is onlygenerated by the magnetic response of the nanoparticles to f₁, andtherefore at these frequencies there is no disturbance signal componentcaused by any coil imbalance. Thus, by extracting these components fromthe overall signal received by the sense coils, these frequencycomponents can be used to detect the presence of the nanoparticles anddistinguish them from disturbance caused by magnetic imbalance of thecoils whether caused by imperfections in construction orthermally-induced distortions.

Further, the drive field from the probe will induce eddy currents in anyelectrically conducting object, e.g. metallic surface that issufficiently close, and the magnetic field created by the eddy currentsmay then be picked up by the sense coils as an extraneous signal.However, as with the imbalances described above, there is no frequencymixing with the signals produced by eddy currents. Thus, the mixed ormodulated frequencies f₀±2nf₁ do not show any component of fieldsgenerated by eddy currents, which means that non-ferromagnetic materialscan be distinguished from nanoparticles. Thus, by extracting thesecomponents from the overall signal received by the sense coils, these f₁frequency components can be used to detect the presence of thenanoparticles and distinguish them from non-ferromagnetic materials.Because the nanoparticles generate side lobes, the existence of sidelobes are indicative of nanoparticles and one such measure of thepresence of nanoparticles is obtained by measuring the amplitude of theside lobe. The greater the side lobe amplitude, the more nanoparticlesare being determined. To adjust for noise in the signals, it is possibleto normalize the measurement of the presence of nanoparticles by usingthe ratio of the amplitude A_(f0±2nf1) of a side lobe to the amplitudeA_(f0) of the fundamental (A_(f0±2nf1)/A_(f0)) where n is a wholenumber.

An additional benefit of making the system insensitive to eddy currentsis that the primary drive frequency can be increased to take advantageof the increased sensitivity of the magnetic nanoparticles at higherfrequencies. For example, the frequency could be increased tosignificantly more than 10 kHz; e.g., to 50 kHz or 100 kHz or 250 kHz ormore. However, ferromagnetic materials will exhibit both eddy currentand magnetic responses, and have similar non-linear magnetic propertiesto the nanoparticles. Hence, the magnetic part of the response is notreadily distinguishable from the magnetic nanoparticles.

Because the magnetic nanoparticles have a non-linear frequency response,the response of the particles can be distinguished from the diamagneticeffect of the body, which has a linear effect. Therefore, the presentinvention can also be used to screen out the diamagnetic effect.

The use of a secondary drive field is suitable for use with any of thecoil embodiments and, for any given embodiment, the detection of themagnetic field will be much less sensitive to imbalances in the coils.This system does not require a balanced coil arrangement, provided thatthe fundamental frequency can be filtered out effectively, althoughpractically this may be desirable for other reasons, e.g. to avoidsaturating the input electronics.

The frequency of the secondary drive field is chosen to providesufficient frequency separation from the primary drive frequency band.This may be, for example, in the region of 0.5% to 10% of the primaryfrequency and preferably in the range of 1% to 5%. For a primaryfrequency of 100 kHz, a secondary frequency of 10 Hz to 10 kHz, and morepreferably between 100 Hz and 1 kHz, could be used. For example, in oneembodiment the primary frequency is 100 kHz and the secondary 1 kHz.Advantageously, the secondary frequency may be chosen to be a multipleof the power supply frequency, e.g. n×50 or n×60 Hz, such that thesecondary drive can be derived from power supply frequency, but thepower supply frequency does not interfere with the sensing. For example,the primary frequency may be 10 kHz and the secondary frequency 200 Hz.Further, a resonant drive circuit may be used for generating the primaryfield to maximize the magnetic field strength for a given level of powerinput. For clinical applications like sentinel lymph node biopsy, asecondary field strength in the region of interest of at least 15microTesla, and preferably greater than 25 microTesla, is appropriate.

In one embodiment, the secondary field is generated by the handheldprobe containing the sensing and primary drive fields, for example byhaving a moving permanent magnet. The movement of the magnet can be, forexample, an oscillatory, rotating or vibratory movement. Alternatively,in one embodiment, an additional coil is added to the probe and isdriven so as to create a time varying magnetic field. In anotherembodiment, the secondary field is generated away from the probe, forexample by means of a device placed near the patient. In anotherembodiment, the field is generated in a pad that sits underneath thearea of the patient that is being sensed. In this case, a coil diameterof at least 200 mm is desirable and a field strength of 2.5 mT (or Hfield of 2000 A/m) is desirable at the center of the coil to providesufficient secondary field strength in the region of interest where theprobe will be used.

In more detail, and referring to FIG. 9, an alternating secondary fieldcan be generated by rotating a permanent magnet 50, 50′ using a motor 54rotating at the desired frequency. The rotating permanent magnet has theadvantage that it is contained within the handheld probe. Such a smallmotor 54 is capable of spinning a rare earth magnet 50 such that themagnetic polarity of the magnet changes with each rotation.Alternatively, and referring to FIG. 10, the secondary field may begenerated electromagnetically by a drive coil 60 located at the rear ofthe handheld probe driven by a signal generator.

It should be understood that the order of steps or order for performingcertain actions is immaterial so long as the present teachings remainoperable. Moreover, two or more steps or actions may be conductedsimultaneously.

It is to be understood that the figures and descriptions of theinvention have been simplified to illustrate elements that are relevantfor a clear understanding of the invention, while eliminating, forpurposes of clarity, other elements. Those of ordinary skill in the artwill recognize, however, that these and other elements may be desirable.However, because such elements are well known in the art, and becausethey do not facilitate a better understanding of the invention, adiscussion of such elements is not provided herein. It should beappreciated that the figures are presented for illustrative purposes andnot as construction drawings. Omitted details and modifications oralternative embodiments are within the purview of persons of ordinaryskill in the art.

The invention may be embodied in other specific forms without departingfrom the spirit or essential characteristics thereof. The foregoingembodiments are therefore to be considered in all respects illustrativerather than limiting on the invention described herein. Scope of theinvention is thus indicated by the appended claims rather than by theforegoing description, and all changes which come within the meaning andrange of equivalency of the claims are intended to be embraced therein.

TABLE 1 Macor ® glass Shapal-M ® Silicon PEEK Zerodur ® ceramic (BN/AlN)carbide CA30 Thermal expansion coefficient, α 0.02-0.1 7-9 4.4 4 25(10⁻⁶/° C.) Thermal conductivity k, (W/mK) 1.46 1.46 90 120 0.92Density, ρ (g/cc) 2.53 2.52 2.95 3.17 1.41 Specific heat capacity, Cp(J/kgK) 820 790 480 1000 1800 Thermal diffusivity, a (×10⁻⁶ m²/s) = 0.700.73 63.56 37.85 0.36 k/ρC_(p) Youngs modulus, E (GPa) 90 66.9 186 4107.7 Fracture toughness K_(ic) (MPa m^(0.5)) 0.9 1.53 2.6 4 ToughnessG_(ic) ≈ K_(ic) ²/E (×10⁻³ KJm⁻²) 9 35 36 39

TABLE 2 Co-located coils Staggered coils 1 Staggered coils 3 Probe OD(mm) 16 18.5 18.5 Coil former OD (mm) 12 15 15 Coil arrangement, fromtip (S = sense, S/D-S/D S/D-S/D S/D-S/D D = drive) See FIG. 1 See FIG. 2See FIG. 2 Drive turns on each coil (layers × 56, AWG28 60 AWG28 60AWG28 turns) (0.32 mm) (0.32 mm) (0.32 mm) Sense turns on each coil 50,AWG38 144 AWG38 108 AWG38 (0.10 mm) (0.10 mm) (0.10 mm) Coil formermaterial Macor ® Shapal-M ® soft Shapal-M ® soft Spacing between twosets of coils 32 58 58 (mm) Relative far field sensing distance 0.721.22 1.15 (Normalised to END-001) Warm up drift over 1 hour (% of END-13% (30 mins) 58% 58% 001 value) Signal change on contact with warm 200%(5 mins)  86% 86% body at 37° C. over 30 mins (% of END- 001 value)

What is claimed is:
 1. A probe for detecting a magnetic markercomprising: a probe core having a first end and a second end, the probecore defining two regions for containing coils of wire, one of theregions being adjacent the first end of the cylindrical probe core; twosense coils, one each of the sense coils being located in a respectiveone of the regions; and two drive coils, one each of the drive coilsbeing located in a respective one of the regions, wherein the regionsare separated by a distance equal to or greater than the diameter of oneof the coils, wherein the regions of the probe define four channels forcontaining coils of wire, a respective two of the channels being locatedin each of the respective regions, wherein two of the channels arelocated adjacent the first end of the cylindrical probe core, whereinone each of the sense coils being located in a respective one of thechannels in each one of the regions, wherein one each of the drive coilsbeing located in a respective one of the channel in each one of theregions, wherein no two coils occupy the same channel, wherein the drivecoils are connected in series, and wherein the sense coils are connectedin anti-series.
 2. The probe of claim 1 wherein the order of the coilsfrom the first end toward the second end of the probe is sense coil,drive coil, sense coil, and drive coil.
 3. The probe of claim 1 whereinthe order of the coils from the first end toward the second end of theprobe is drive coil, sense coil, drive coil, and sense coil.
 4. Theprobe of claim 1 wherein the order of the coils from the first endtoward the second end of the probe is sense coil, drive coil, drivecoil, and sense coil.
 5. The probe of claim 1 wherein the order of thecoils from the first end toward the second end of the probe is drivecoil, sense coil, sense coil, and drive coil.
 6. A probe for detecting amagnetic marker comprising: a probe core having a first end and a secondend, the probe core defining two regions for containing coils of wire,one of the regions being adjacent the first end of the cylindrical probecore; two sense coils, one each of the sense coils being located in arespective one of the regions; and two drive coils, one each of thedrive coils being located in a respective one of the regions, whereinthe regions are separated by a distance equal to or greater than thediameter of one of the coils, wherein the regions of the probe definefour channels for containing coils of wire, a respective two of thechannels being located in each of the respective regions, wherein two ofthe channels are located adjacent the first end of the cylindrical probecore, wherein one each of the sense coils being located in a respectiveone of the channels in each one of the regions, wherein one each of thedrive coils being located in a respective one of the channel in each oneof the regions, wherein no two coils occupy the same channel, whereinthe drive coils are connected in anti-series, and wherein the sensecoils are connected in series.
 7. The probe of claim 6 wherein the orderof the coils from the first end toward the second end of the probe issense coil, drive coil, sense coil, and drive coil.
 8. The probe ofclaim 6 wherein the order of the coils from the first end toward thesecond end of the probe is drive coil, sense coil, drive coil, and sensecoil.
 9. The probe of claim 6 wherein the order of the coils from thefirst end toward the second end of the probe is sense coil, drive coil,drive coil, and sense coil.
 10. The probe of claim 6 wherein the orderof the coils from the first end toward the second end of the probe isdrive coil, sense coil, sense coil, and drive coil.